At which temperature is a standard hydrogen electrode defined?

The standard hydrogen electrode (SHE) is defined at a temperature of 298K, which corresponds to 25 degrees Celsius. This temperature is significant because it is commonly used as the standard condition for thermodynamic measurements in electrochemistry. The SHE serves as a reference electrode with a defined reduction potential of 0.00 volts, and all other electrode potentials are measured against it under these standard conditions, which also include a pressure of 1 atmosphere for gases and a concentration of 1 mol/L for solutions.

The choice of 298K allows for consistency and comparability of electrochemical data across different studies and applications. It is a temperature that is practical for laboratory conditions, making it a standard in electrochemical experiments. Other temperatures, such as those listed in the question, do not serve as the defined standard for the SHE, as they would result in different thermodynamic properties and potential values.